Systemd unit 的所有状态
Systemd 管理的资源的单位是 unit，最常见的 unit 就是一个服务。我们经常使用 systemctl status 查看某些服务的状态，来确认这个服务的运行是否正常。像下面这样：
➜ ~ $ systemctl status docker
● docker.service - Docker Application Container Engine
Loaded: loaded (/lib/systemd/system/docker.service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2018-09-13 11:53:33 CST; 1 day 2h ago
Docs: https://docs.docker.com
Main PID: 1917 (dockerd)这里的 Loaded 和 Active 两行就表示这个 unit 的状态，其中 Loaded 表示这个 unit 是否已经正确读到内存之中，Active 则表示这个 unit 的运行状态。另外，Active 这里包含了一个 state 和一个 substate，括号内的是 substate，state 的状态定义对所有 unit 都是一样的，但是不同类型的 unit 会定义不同的 substate （systemd 有 11 种类型的 unit，具体请查看 man page）。
那么一个 unit 到底有哪些状态呢？我发现我在 man page 中找不到所有状态的说明，但是有个神奇的命令却能列出所有的状态：
systemctl --state=help
Available unit load states:
stub
loaded
not-found
error
merged
masked
Available unit active states:
active
reloading
inactive
failed
activating
deactivating
Available automount unit substates:
dead
waiting
running
failed
Available device unit substates:
dead
tentative
plugged
Available mount unit substates:
dead
mounting
mounting-done
mounted
remounting
unmounting
remounting-sigterm
remounting-sigkill
unmounting-sigterm
unmounting-sigkill
failed
Available path unit substates:
dead
waiting
running
failed
Available scope unit substates:
dead
running
abandoned
stop-sigterm
stop-sigkill
failed
Available service unit substates:
dead
start-pre
start
start-post
running
exited
reload
stop
stop-sigabrt
stop-sigterm
stop-sigkill
stop-post
final-sigterm
final-sigkill
failed
auto-restart
Available slice unit substates:
dead
active
Available socket unit substates:
dead
start-pre
start-chown
start-post
listening
running
stop-pre
stop-pre-sigterm
stop-pre-sigkill
stop-post
final-sigterm
final-sigkill
failed
Available swap unit substates:
dead
activating
activating-done
active
deactivating
deactivating-sigterm
deactivating-sigkill
failed
Available target unit substates:
dead
active
Available timer unit substates:
dead
waiting
running
elapsed
failed
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